Diesel engines, unlike gasoline engines, do not use spark plugs to induce combustion. Rather, diesel engines rely on compression to raise the temperature of air in the cylinder (taken in during the intake stroke) to a temperature such that, when diesel fuel is introduced to the hot compressed air, the diesel fuel will spontaneously combust. When the piston reaches the top of its travel path, or near top dead center, a fuel mist is typically injected into the cylinder, which instantly combusts, forcing the piston downwards and generating power.
In a cold environment, particularly if a diesel engine has not been running for several hours, the engine block similarly becomes cold and, when attempting to start the engine, acts as a heat sink, quickly dissipating the heat generated by the pistons compressing air. As a result, the engine may be unable to start because it cannot generate and maintain enough heat to ignite the diesel fuel.
Compounding this problem is the fuel itself. During cold start-up, the diesel fuel becomes more difficult to ignite. Without being bound to a particular theory, during cold weather starting, all of the diesel fuel may not evaporate. As temperature decreases, the viscosity of the diesel fuel may be understood to increase, which adversely influences the spray pattern when the fuel is injected into the cylinder. Furthermore, the vapor pressure of diesel fuel also decreases with decreasing temperature making the fuel less volatile. Thus, upon injection, a portion of the fuel may not ignite. In order to compensate for such loss in fuel, the quantity of fuel injected may be typically increased. As such, the engine may be required to run more rich during start-up, and produce higher emissions in the form of unburned hydrocarbons.
A glow plug is a heating device which is typically used to aid the starting of diesel engines. A glow plug includes a heating element at the tip which, when electrified, heats by means of electrical resistance. Each cylinder of the diesel engine typically includes a glow plug. Heat generated by a glow plug promotes ignition of the fuel mist near the glow plug tip which leads to increased in-cylinder temperature and subsequent combustion of the rest of the fuel.
Glow plugs are typically activated for a time period (e.g. few seconds) before attempting to start the engine, which may be referred to as a “pre-glow” period. In addition to helping with engine starting, the glow plugs may remain on while the engine warms up (i.e. post starting) to a predetermined temperature, which may be referred to as “post-glow” period or “after-glow” period. Such use of the glow plugs while the engine warms up may improve combustion stability and/or reduce harmful exhaust emissions, such as unburned hydrocarbons (HC). However, such use may also increase fuel consumption.
For example, by glowing the glow plugs from a few seconds before engine start to an engine temperature of 40° C., hydrocarbon emissions may be decreased. However, use of the glow plugs for such a duration may increase the power consumption to the glow system, as well as fuel consumption, leading to an increase in carbon dioxide production. Thus, a balance of competing interests should be considered when using glow plugs.
Modern glow plugs are capable of temperatures of up to 1300° C., with the temperature thereof being a function of the supplied power (e.g. voltage). Determination and application of the power level for glow plugs during engine warm-up generally varies by vehicle manufacturer. Some manufacturers may adjust power to the glow plugs during warm-up simply as a function of engine speed. Other manufactures may adjust power to the glow plugs during warm-up as a function of engine speed and pedal position. For both of these approaches, the applied power (voltage) may be determined solely from a look-up table.
The selection of the applied voltage from the look-up table may be simply based on the resistance of the glow plug. In other words, by knowing the electrical resistance of the glow plug, and the power applied to the glow plug, a theoretical temperature for the glow plug may be determined. However, while glow plug temperature may be a function of the applied power level, unaccounted for secondary factors may lead to significant deviations from the theoretical temperature of the glow plug. Ignoring these secondary factors may lead to glow plug temperature levels well outside targeted ranges, such as below the maximum temperature available, over a large engine operation area which limits the potential benefit from the glow plugs to improve combustion stability and/or reduce harmful exhaust emissions. What is needed is a glow plug control apparatus which more accurately models power to be supplied to the glow plug(s), based on a number of engine operating variables, as a means to better indirectly control the resulting glow plug temperature.